COPPER ALLOYS

20.2 COPPER ALLOYS

Tin bronzes are copper – tin alloys noted for their high strength. Alloys containing more than 5% Sn are especially resistant to impingement attack. The copper – silicon alloys containing 1.5 – 4% Si have better physical properties than copper and similar general corrosion resistance. Seawater immersion tests at Panama showed that 5% Al − Cu was the most resistant of the common copper - base alloys, losing only 20% of the corresponding weight loss of copper after 16 years [17] .

20.2.1 Copper – Zinc Alloys (Brasses)

Copper – zinc alloys have better physical properties than copper alone, and they are also more resistant to impingement attack; hence, brasses are used in prefer- ence to copper for condenser tubes. Corrosion failures of brasses usually occur by dezincifi cation, pitting, or stress - corrosion cracking. The tendency for brass to corrode in these ways, except for pitting attack, varies with zinc content, as shown in Fig. 20.3 . Pitting is usually caused by differential aeration cells or high - velocity conditions. It can normally be avoided by keeping the brass surface clean at all times and by avoiding velocities and design geometry that lead to impingement attack.

Various names have become attached to brasses of different zinc content. Muntz metal , 40% Zn − Cu, is used primarily for condenser systems that use fresh water (e.g., Great Lakes water) as coolant. Naval brass is a similar composition, but containing 1% Sn. Manganese bronze is also similar, containing about 1%

372 COPPER AND COPPER ALLOYS

Figure 20.3. Trends of dezincifi cation, stress - corrosion cracking, and impingement attack with increasing zinc content in copper – zinc alloys (brasses).

each of tin, iron, and lead; it is used for ship propellers, among other applications. Dezincifi cation of manganese bronze propellers in seawater is avoided to some extent by the cathodic protection afforded by the steel hull.

Yellow brass , 30% Zn − Cu, is used for a variety of applications where easy machining and casting are desirable. The alloy gradually dezincifi es in seawater and in soft fresh waters. This tendency is retarded by addition of 1% Sn, the cor- responding alloy being called admiralty metal or admiralty brass . Addition of small amounts of arsenic, antimony, or phosphorus still further retards the rate of dezincifi cation; the resulting alloy, called inhibited admiralty metal , is used in seawater or fresh - water condensers.

Red brass , 15% Zn − Cu, is relatively immune to dezincifi cation, but is more susceptible to impingement attack than yellow brass.

20.2.2 Dealloying/Dezincifi cation

Dezincifi cation, as one type of dealloying, was defi ned in Section 2.4 . Other types of dealloying include the selective dissolution of copper in copper – gold alloys.

In brasses, dezincifi cation takes place either in localized areas on the metal surface, called plug type (Fig. 20.4 ), or uniformly over the surface, called layer type (Fig. 20.5 ). Brass so corroded retains some strength, but has no ductility. Layer - type dezincifi cation in a water pipe may lead to splitting open of the pipe under conditions of sudden pressure increase; and, for plug type, a plug of dez- incifi ed alloy may blow out, leaving a hole. Because dezincifi ed areas are porous,

COPPER ALLOYS

Figure 20.4. Plug-type dezincifi cation in brass pipe (actual size).

Figure 20.5. Layer-type dezincifi cation in brass bolts (actual size).

374 COPPER AND COPPER ALLOYS

plugs may be covered on the outside surface with corrosion products and residues of evaporated water.

Conditions of the environment that favor dezincifi cation are (1) high tem- peratures, (2) stagnant solutions, especially if acid, and (3) porous inorganic scale formation. Looking at the situation metallurgically, brasses that contain 15% or less zinc are usually immune. Also, dezincifi cation of α - brasses (up to 40% Zn) can be reduced, as mentioned earlier, by small alloying additions of tin plus a few hundredths percent arsenic, antimony, or phosphorus.

Although inhibited admiralty brass resists dezincifi cation, conditions favor- ing thermogalvanic action between overheated local areas and adjoining colder areas of heat exchanger tubes can cause attack of this kind at the overheated zones. Attack is reduced by using inhibited or scaling (positive saturation index) waters [18] .

The detailed mechanisms of dealloying have recently been reviewed [19] . The four main mechanisms that researchers have developed to account for the pro- cesses by which one metal in an alloy is removed by corrosion, leaving behind a porous metal, are:

1. The ionization – redeposition mechanism , according to which the alloy cor- rodes and the more noble metal (copper in copper – zinc alloys) is then redeposited to form a porous outer layer [20] .

2. The volume diffusion mechanism , based on selective dissolution of the less noble element and volume diffusion of both elements in the solid phase [21, 22] .

3. The surface diffusion mechanism , in which only the less noble element (zinc in copper – zinc alloys) dissolves and the remaining more noble metal is rearranged by diffusion on the surface and nucleation of islands of almost pure metal [23] .

4. The percolation model of selective dissolution , an extension of the surface diffusion mechanism, based on preexisting interconnected paths of like elements in the binary alloy and effects of curvature on dissolution poten- tial [24, 25] .

Any one of these mechanisms may apply in specifi c instances of dealloying. For example, twin bands in brass, visible in the completely or incompletely dezincifi ed layer, constituted early evidence for a volume diffusion mechanism of zinc trans- port from the bulk alloy to the surface [26] . In the gold – copper alloy system, copper corrodes preferentially, without dissolution of gold, leaving a porous residue of gold – copper alloy or pure gold.

20.2.3 Stress-Corrosion Cracking (Season Cracking)

Virtually all copper alloys, as well as pure copper, can be made to crack in ammonia [27] . Requirements for cracking include the presence of water, ammonia,

COPPER ALLOYS

air or oxygen, and tensile stress in the metal. Only trace amounts of ammonia are required in many cases, and cracking occurs at ambient temperature. Stress -

corrosion cracking failures of copper pipe under elastomeric insulation have been attributed to wet ammoniacal environments [27, 28] . Small concentrations of hydrogen sulfi de inhibit S.C.C. of brass in petroleum refi nery process streams, most likely by reducing the dissolved oxygen concentration [27, 29] .

When an α brass is subjected to an applied or residual tensional stress in contact with a trace of NH 3 or a substituted ammonia (amine), in the presence of oxygen (or another depolarizer) and moisture, it cracks usually along the grain boundaries (intergranular) (see Fig. 20.6 ). Cracking through the grains (trans- granular) may occur in specifi c test solutions or if the alloy is severely deformed plastically.

According to one source, both types of cracking were originally called season cracking because of the resemblance of stress - corrosion cracks in bar stock to those of seasoned wood. In England, the origin of the term is ascribed to the fact that years ago brass cartridge cases stored in India were observed to crack, par- ticularly during the monsoon season.

Traces of nitrogen oxides may also cause stress - corrosion cracking, probably because such oxides are converted to ammonium salts on the brass surface by chemical reaction with the metal. In one instance of this kind, premature failure of yellow brass brackets in the humidifi er chamber of an air - conditioning system was traced to this cause [30] . The air had passed through an electrostatic dust

Figure 20.6. Intergranular stress -corrosion cracking of brass (75 ×) (specimen stored 1 year).

376 COPPER AND COPPER ALLOYS

precipitator, the high voltage fi eld of which generated traces of nitrogen oxides. These, in turn, formed corrosion products on the brass surface which were found

by analysis to contain a high proportion of NH 4 , causing intergranular cracking of the stressed brackets. Similar cracking of stressed brass could be reproduced in the laboratory over a period of days by using a spark discharge in air of 100% relative humidity.

Stress - corrosion cracking of 12% Ni, 23% Zn − Cu alloy (nickel brass) parts of Central Offi ce Telephone Equipment in Los Angeles occurred within two years for similar reasons [31] . Pollution of Los Angeles air accounts for abnor- mally high concentrations of nitrogen oxides and suspended nitrates, with the latter settling down as dust on the brass parts. Similar failures have not been observed as frequently in New York City, where the air contains less nitrate, but also many more sulfate particles than Los Angeles air, indicating that sulfates may act as inhibitors.

Mattsson [32] observed that minimum cracking time of 37% Zn − Cu brass in

a solution of 1 mole NH 3 + NH + 4 , plus 0.05 mole CuSO 4 per liter occurred at pH

7.3, with times to failure being longer at higher pH and considerably longer at lower pH values. Johnson and Leja [33] reported stress - corrosion cracking of brass in alkaline cupric citrate and in tartrate solutions at pH values in which complexing of Cu 2+ is pronounced.

Figure 20.7 shows the effect of applied potential [34] on S.C.C. of 37% Zn − Cu brass in a solution similar to that proposed by Mattsson. The critical potential below which cracking does not occur is 0.095 V (S.H.E.). Since the corresponding corrosion potential is 0.26 V, S.C.C. on simple immersion of brass is spontaneous. Stress - corrosion cracking also occurs on substituting Cd 2+ or Co 2+ in place of Cu 2+ additions, but at more noble potentials. It is found that, in the absence of other metal ammonium complexes, the equivalent of more than 0.003 M Cu 2+ must be

present in the test solution for spontaneous cracking to occur. Also, addition of more than 0.005 M NaBr or 0.04 M NaCl to the Mattsson test solution inhibits

spontaneous cracking. The latter observations are related to a shift of potentials by Cu 2+ , Br − , or Cl − such that, when cracking does not occur, the corrosion poten- tial in each case lies active to the critical potential for S.C.C. Annealed brass, if not subject to a high applied stress, does not stress - corro- sion crack. Whether residual stresses in cold - worked brass are suffi cient to cause stress - corrosion cracking in an ammonia atmosphere can be checked by immers-

ing brass in an aqueous solution of 100 g mercurous nitrate [Hg 2 (NO 3 ) 2 ] and

13 mL nitric acid (HNO 3 , specifi c gravity 1.42) per liter of water. Mercury is released and penetrates the grain boundaries of the stressed alloy. If cracks do not appear with 15 min, the alloy is probably free of damaging stresses.

No alloying additions in small amounts effectively provide immunity to this type of failure in brasses. Low - zinc brasses are more resistant than high - zinc brasses.

High - zinc brasses (e.g., 45 – 50% Zn − Cu) having a β or β + γ structure, stress - corrosion crack through the grains (i.e., transgranularly); and, unlike α brasses, only moisture is required to cause failure [35] .

COPPER ALLOYS

Figure 20.7. Effect of applied potential on time to failure of 37% Zn − Cu brass in 0.05 M CuSO 4 , CdSO 4 , or CoSO 4 , 1 M (NH 4 2 ) SO 4 , pH 6.5 at room temperature [34] . ( Reproduced with permission. Copyright 1975, The Electrochemical Society .)

The mechanism of stress - corrosion cracking in brasses has been the subject of much study. Both high - purity alloys and single crystals of α brass crack when stressed in NH 3 atmospheres [36] . In support of an electrochemical mechanism, it has been noted that the grain boundaries of polycrystalline brasses are more active in potential than the grains, as measured in NH 4 OH, but not in FeCl 3 solu- tion, in which stress - corrosion cracking does not occur [37] . It has been proposed, alternatively, that a brittle oxide fi lm forms on brasses that continuously fractures under stress exposing fresh metal underneath to further oxidation [38] . Accord- ing to the generally accepted fi lm rupture mechanism, S.C.C. initiates by fi lm rupture as a result of plastic deformation at the crack tip. The crack propagates by localized anodic dissolution, and repassivation is inhibited by further plastic deformation at the crack tip [39 – 41] .

There is evidence, in any event, that zinc atoms aided by plastic deformation segregate preferentially at grain boundaries. The resulting composition gradient favors galvanic action between such areas and the grains, accounting for slow

378 COPPER AND COPPER ALLOYS

intergranular attack in a variety of corrosive media without the necessity of an applied stress (intergranular corrosion). But such areas, subject to plastic defor- mation, may also favor adsorption of complex ammonium ions within a specifi c potential range, leading to rapid crack formation. Similar effects can occur along slip bands (transgranular cracking). Although segregated zinc may be essential to the observed intergranular corrosion of brasses, it is probable that the defect structure of grain boundaries or of slip bands is more important to stress - corrosion cracking. Hence, failure of copper - base alloys by cracking can occur when copper is alloyed not only with zinc, but also with a variety of other ele- ments, such as silicon, nickel, antimony, arsenic, aluminum, phosphorus [42] , or beryllium [43] .

To explain the mechanism of transgranular S.C.C. in α brass, Newman and Sieradzki have proposed the fi lm - induced cleavage model, according to which a brittle crack that initiates in a thin surface fi lm propagates into the ductile matrix and eventually blunts and arrests, after which the process repeats itself [39, 44, 45] .

Susceptibility to stress - corrosion cracking of brasses can be minimized or avoided by four main procedures:

1. Stress - Relief Heat Treatment. For 30% Zn − Cu brass, heating at 350 ° C (660 ° F) for 1 h may be effective, but recrystallization and some loss of strength of the alloy result. Because ammoniacal S.C.C. can occur at rela- tively low stress levels, stress relief may not be adequate in all cases [27] .

2. Avoiding Contact with NH 3 (or with O 2 and Other Depolarizers in the

It is diffi cult to guarantee that there will be no contact with NH 3 , because of the small traces of ammonia that cause cracking. Plastics containing or decomposing to traces of amines are a continuing source of damage to unannealed brass. Fertilizer washing from farm land, or air over fertilized soil, has similarly caused cracking of brass. On the other hand, brass condenser tubes do not crack in boiler - water condensate

Presence of NH 3 ).

containing NH 3 , because the concentration of oxygen is extremely low.

3. Cathodic Protection. Cathodic protection to a potential below the critical value can be provided by either impressed current, or by coating brass with a sacrifi cial metal (e.g., zinc).

4. Using H2S as an Inhibitor [27, 29] . The mechanism may, in part, involve reaction with available free oxygen.

20.2.4 Condenser Tube Alloys Including Copper –Nickel Alloys

For fresh waters, copper, Muntz metal, and admiralty metal (inhibited) are fre- quently used. For brackish or seawater, admiralty metal, one of the cupro - nickel alloys (10 – 30% Ni, bal. Cu) and aluminum brass (22% Zn, 76% Cu, 2% Al, 0.04% As) are used. For polluted waters, cupro - nickel alloys are preferred over alumi- num brass because the latter is subject to pitting attack. Aluminum brass may also pit readily in unpolluted, but stagnant, seawater.

REFERENCES

Aluminum brass resists high - velocity waters (impingement attack) better than does admiralty metal. Cupro - nickel alloys are especially resistant to high - velocity seawater when they contain small amounts of iron and sometimes man- ganese as well. For the 10% Ni cupro - nickel alloy, the optimum iron content is about 1.0 – 1.75%, with 0.75% Mn maximum; for the analogous 30% Ni composi- tion, the amount of alloyed iron is usually less (e.g., 0.40 – 0.70% Fe accompanied by 1.0% Mn maximum) [46] . It is found that supplementary protective fi lms are formed on condenser tube surfaces when iron is contained in water as a result of corrosion products upstream or when added intentionally as ferrous salts. Accordingly, the benefi cial effect of iron alloyed with copper – nickel alloys is considered to result from similar availability of iron in the formation of protective fi lms.

The susceptibility of Cu − Ni alloys to corrosion increases in aerated sulfi de - polluted seawater. It appears that sulfi des interfere with formation of the usual protective oxide fi lms [47] . It is pertinent that the 30% Ni − Cu alloy is relatively resistant to stress - cor- rosion cracking compared to the 10 or 20% Ni − Cu alloys [42] , or compared to any of the 30% Zn − Cu brasses. A detailed general account of the behavior of copper – nickel alloys, especially the 10% Ni − Cu alloy, in seawater is given by Stewart and LaQue [48] .